1. Field of the Invention
The present invention relates to a charging apparatus. More specifically, the present invention relates to a novel charging apparatus which is suitable for a rapid charge of nickel-metal hydride secondary battery (hereinafter, simply called as "Ni-MH battery".
2. Description of Prior Arts
Conventionally, various kinds of systems for rapidly charging a secondary battery have been proposed and put into practical use. A first system is disclosed in, for example, Japanese Patent Laying-Open No. 59-6731 laid-open on Jan. 13, 1984. In the first system, by utilizing a matter that a battery temperature increases with a progress of a charge, the charge is stopped as regarded as a fully charged state when the battery temperature reaches a predetermined value. In the first system, since the battery temperature is easily influenced by an ambient temperature, it is impossible to accurately control the charge.
A second system is a so-called "-.DELTA.V system" which is disclosed in, for example, U.S. Pat. No. 4,354,148 issued on Oct. 12, 1982 or U.S. Pat. No. 4,387,332 issued on Jun. 7, 1983 both of which were assigned to the same assignee as that of the present invention. A battery voltage of a nickel-cadmium (Ni-Cd) battery changes as shown in FIG. 1 with a progress of a charge. Although the battery voltage indicates a peak thereof at a timing X in FIG. 1, the Ni-Cd battery is not fully charged at this timing X and becomes a fully charged state at a timing Y when the battery voltage slightly drops while passing the peak. Therefore, in the second system, the charge is stopped at the timing Y in FIG. 1. The second system is suitable for a Ni-Cd battery. However, in a Ni-MH battery to which the present invention is directed, although a battery voltage changes in a similar manner as that of the Ni-Cd battery, the timing X (FIG. 1) when the battery voltages reaches the peak is a fully charged state. Therefore, if the charge is continued after the battery voltage reaches the peak in accordance with the second system as similar to the Ni-Cd batteries, there is a problem that deterioration due to heat generation according to a chemical reaction inside the Ni-MH battery occurs. Therefore, the second system is not suitable for a charging apparatus of the Ni-MH battery.
A third system is disclosed in UK Patent No. 1,097,451 issued on Jan. 3, 1968. In the third system, an increasing rate of a battery voltage with respect to a time at a fully charged state is measured in advance and, by utilizing the increasing rate obtained through the measurement as a reference, in practically charging, the charge is stopped when the increasing rate of the battery voltage becomes equal to or below the reference value. The third system can be utilized as a charging apparatus for a Ni-MH battery in which the characteristic deterioration due to overcharge becomes a large problem; however, in the third system, there is a disadvantage that flexibility lacks because the above described increasing rate must be practically measured at every timing when a kind of battery, battery capacitance, residual capacitance, charging current (rapid charge or normal charge) or the like is changed.